1. Field of the Invention
Example embodiments of the present invention are generally related to detection of discontinuous transmission frames in transmitted data, and to a method of generating a signal metric for use in DTX detection.
2. Description of Related Art
Third generation wireless standard 3GPP2-CDMA2000-1x is designed for both voice and data applications. Typically, transmission from a base station to a mobile station in a wireless communication system is known as a forward link, and transmission from the mobile station to the base station is known as a reverse link. When a system is used for data applications, reverse link channels required to support the application usually involve a dedicated control channel (R-DCCH), which is used to transmit control information, and a supplemental channel (R-SCH), which is used to transmit data. These channels are in addition to a reverse link pilot channel, which is always transmitted.
Due to the bursty nature of data applications, if the transmitter signal is switched on only during periods of data input, the duty cycle of the mobile station can be cut to less than 50 percent in some applications. Thus to extend mobile station battery life and to reduce interference to other users, discontinuous transmission (DTX) can be used. DTX is a method of momentarily powering-down, or muting, a mobile station when there is no data input to the transmitter. A mobile station on its own discretion decides whether to send a packet of data to the base station on a frame-by-frame basis. The mobile station decides not to send a packet of data to extend the battery life of the mobile station battery life and reduce interferences in a radio environment. DTX is used when there is no data to transmit on either channel. In other words, no signal is actually transmitted during DTX frames of a particular channel.
In the conventional art, a mobile station (or user equipment (UE)) does not notify a base station that it has sent a frame without any symbols (data), i.e., a DTX frame. The base station (or Node-B) makes that determination on its own.
An issue with DTX transmission is its impact on power control. A base station receives a checksum value, which is typically included at an end of a frame. Cyclic redundancy checking (CRC) checksum at a base station receiver is used to drive an outer-loop power control, so that a pre-defined frame error rate (FER) may be achieved. As explained above, the base station does not know that the mobile station has sent a frame without any data, so it processes the frame as if there is data transmitted. This may result in a CRC error since no signal is actually transmitted in that frame. This false CRC error may drive up the outer-loop power control target, which in turn increases interference level to other users and wastes power on a mobile station transmitter. Therefore, a base station receiver must detect whether a DTX frame is present, so that an outer-loop power control can either ignore a data frame CRC report or uses some other metric, (such as pilot frame error defection) to drive the outer-loop power control.
Another type of checksum error may occur when a transmitted frame becomes distorted during transmission due to poor channel conditions. Here, the base station transmits a frame but the transmitted frame is not properly received by the base station. This type of error is known as an “erasure.”
FIGS. 1A and 1B is a block diagram to illustrate Reverse link Dedicated Control Channel (R-DCCH) or Reverse link Supplement Channel (R-SCH) processing employing a conventional DTX detector. The blocks shown for the transmitter 100 at the UE and blocks at base station receiver 150 represent processing functions performed by software routines which are iterated by respective processors at the UE or Node-B respectively.
Referring to FIG. 1A, at the UE transmitter 100, a data packet or frame (i.e., DCCH, and/or SCH data) is appended with CRC bits at CRC append unit 105, forward error code (FEC) encoded at FEC coder 110, rate adjusted at rate matching unit 115, interleaved at interleaver 120 and weighted by gains at gain unit 135 to achieve certain power levels. The pilot channel is also weighted by gains at gain unit 140 to achieve certain power levels and then spread by an orthogonal Walsh code at orthogonal spreading unit 140. The two channels are then combined (code-division multiplexed) at multiplexer 145. The multiplexed signal may be scrambled and filtered by a shaping filter (not shown) before being modulated to RF (not shown for purposes of clarity) and sent through the propagation channel 147 to the base station (Node-B) receiver 150.
At the Node-B receiver 150, the received signal 148 first passes a matched filter (not shown for clarity) and is sent to an R-DCCH/R-SCH despreader/demodulator to generate soft symbols for further processing by blocks such as decoder 176 to recover the transmitted data from the frame. The received signal 148 is additionally received by a pilot channel processor 155, which separates the pilot channel from other channels based on its Walsh code and generates channel estimates (shown at 157) and noise energy (shown at 158). The channel estimates 157 are transmitted to the R-DCCH or R-SCH despreader & demodulator 160 to generate the soft symbols (shown at 165) for further processing in an R-DCCH/R-SCH post processor 170 and a DTX detector 180. The noise energy 185 is used for DTX detection on the corresponding data frame by the DTX detector 180.
The R-DCCH or R-SCH post-processing by the R-DCCH or R-SCH post processor 170 may be the reverse processing of that performed at the UE transmitter side 100. The soft symbols 165 output from the R-DCCH/R-SCH despreader & demodulator 160 are de-interleaved at de-interleaver 172, rate de-matched at rate de-matching unit 174, decoded at decoder 176, and CRC checked at CRC check unit 178 to output the frame data and/or determine a CRC pass/fail.
The DTX detector 180 calculates a signal energy in the received frame by accumulating L2-norms in accumulator 184. The L2-norms are determined by a L2-norm calculation unit 182 based on the generated soft symbols 165. Assuming for example that the complex output signal is z=a+j*b, its L2-norm is given by L2(z)=a2+b2. The L2-norms are this accumulated over the frame interval in accumulator 184 to output the signal energy.
The detector 180 then calculates the signal-to-noise energy ratio (SNR) based on the noise energy 158 received from the pilot channel processor 155 and the determined signal energy from accumulator 184 at SNR calculation unit 186. The SNR value is then sent to a comparator 186. If the comparator 188 determines that the SNR is less than some pre-defined threshold 188, the base station receiver 150 determines that the frame is a DTX frame, (DTX On), or not (DTX Off), respectively.
In the conventional DTX detector 180 of FIG. 1B, DTX detection performance is not satisfactory for short data frames (5 ms R-DCCH, or R-SCH with low data rates). In the conventional detector of FIG. 1B, energy is estimated prior to decoding. Therefore, to remove modulation, soft symbols must be squared or the absolute value of the soft symbols must be determined (at L2-norm calculation unit 182) prior to accumulation at accumulator 184 to generate the signal energy. The conventional DTX detector 180 also cannot accurately distinguish whether a checksum error was caused by an erasure or a DTX frame. For larger data frames, e.g., R-SCH with very high data rates, especially if the detector 180 is to be implemented in Digital Signal Processing (DSP) or Field-Programmable Gate Array (FPGA), accumulation typically takes too long.